1. Field of the Invention
The present invention generally relates to disk drives, and particularly to servo control for head positioning using servo data.
2. Description of the Related Art
Heretofore, in a disk drive particularly typified by a hard disk drive, there has been incorporated a servo system in which servo data recorded on a disk medium is used to position a head at a target position on the disk medium. Here, the target position refers to a target track from which data is read out or on which data is written.
Generally, servo data is recorded on a disk medium by a servo track writer in a servo writing step included in a process of manufacturing a disk drive.
In the disk drive, the head is mounted on a rotary actuator and moved in a radial direction of the disk medium. Therefore, particularly in an inner peripheral or outer peripheral area of the disk medium, there arises a so-called skew angle which indicates an inclination of the head with respect to a data track (a track on which data is recorded).
In recent years, in a disk drive, track density on a disk medium has been more and more increased for high recording density. Such a background and the above-mentioned inclination of the head raise the possibility that low quality servo data is recorded in an erase area provided between adjacent data tracks in the servo writing step.
Accordingly, in the disk drive, when a head positioning operation is performed, the read head may read out the low quality servo data as noise from the erase area on the disk medium. The influence by this noise decreases accuracy of the head positioning operation.
In order to solve such a problem, there has been suggested a servo writing method of recording servo data in the inner peripheral area and the outer peripheral area of the disk medium separately instead of a method of concurrent servo writing on the whole area of the disk medium (for example, refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-268516 or No. 2001-89062).
According to the servo writing method described in the literatures of the prior art, normal servo data is consequently overwritten in the erase area between the data tracks. In the erase area, therefore, low quality servo data is inhibited from being recorded.
However, in this method, unrecorded area of servo data equivalent to one to several tracks (referred to as a non-servo range) occurs in a middle peripheral area on the disk medium. Therefore a servo system using the servo data cannot acquire the servo data from the non-servo range, thereby having difficulty with the normal head positioning operation.